This invention relates to a unique sensing device, based upon the resistance-tape principle, which is known commercially and described in the literature under the trademark "Metritape" sensor, and which is the subject of several U.S. Patents, including U.S. Pat. Nos. 3,511,090; 3,583,221; and 3,792,407.
The Metritape sensor comprises an elongated metallic base strip having electrical insulation on the edges and back of the strip to define an uninsulated zone along the length of the base strip,, and a resistance wire or ribbon helically wound around the insulated base strip, with the helical turns bridging the insulated edge portions being spaced from the underlying uninsulated zone of the base strip. This sensor structure is enclosed within a continuous polymeric or other protective sleeve to provide a clean and dry inner chamber for the sensor. The sensor is disposed within a tank or vessel containing the liquid or fluent material, the level of which is to be monitored. The pressure of the material surrounding the immersed sensor causes the deflection of the enclosing sleeve and helical turns in the immersed portion of the sensor into engagement and electrical contact with the underlying base strip, such that an electrical resistance proportional to material level is provided.
Applications for this elongated resistance-tape sensor have ranged from the gauging of short land-based tanks to deep oil and ballast tanks of ocean-going supertankers; from tanks open to atmosphere, to closed heavy-walled tanks containing several atmospheres of pressure; from tanks never filled above 98% of level, to tanks that are frequently overfilled to a height that imparts an overpressure upon tank and sensor components; from tanks containing water and benign liquids to those holding corrosive chemicals and aggressive solvents; also including applications in which the top head of the resistance-tape sensor may be inadvertently or intentionally submerged below the surface of the liquid being gauged.
Within this application range, it is the role of the sensor head to contain and terminate the materials that comprise the sensor upper end, to mechanically support the sensor in its suspended location, to exclude the entry of vapors and liquids from below or above into the inner sensor chamber, to provide means by which pressure within the sensor chamber is equalized to surrounding pressure in the tank, and also to provide means for bringing resistance element and electronic circuits within the sensor out to external connections without providing leakage paths for liquid or vapor entry.
Several techniques have been employed to provide such a multi-purpose top termination for the resistance-tape sensor. In one configuration the sensor top end is enclosed within a metallic tube that may be flared for supporting sensor weight against a threaded nipPle. Elements of the sensor are brought into the underside of the tube, and a breather tube and leadwires are brought through a top plate, or header, out of the top of the enclosing tube. At final assembly, the enclosing tube is inverted and, with parts properly positioned as they enter and leave the tubular structure, an encapsulant such as rigid epoxy is poured into the inverted tube and allowed to harden so as to form a rigid end sealing structure.
Depending upon application circumstances, such an encapsulated head may develop leakage paths due to a number of causes. Adhesion of components to the epoxy may be initially established, but mechanical shock or temperature cycling may cause the parts to shift relative to one another sufficient to develop small leakage paths or planes. In addition, as the epoxy sets up, it is exothermic and causes a rise in temperature of the components being encapsulated. As the epoxy sets and the temperatures diminish, there may be a shrinkage of parts which can cause microscopic leakage paths to open up. Furthermore, if leaks do appear after the setting of the epoxy, it is difficult to locate and seal them reliably, or to recover the sensor components for remaking the encapsulated head seal.
A second approach to sensor top-end termination is to provide a tubular structure at the top to which the outer sheath or boot of a sensor is engaged; and a mechanical clamping structure, such as of polytetsafluoroethylene for chemical corrosion resistance, is used to enclose the top end of the boot and to compress it firmly against the upper tubular structure. Such mechanical clamping has the advantage that it can be tightened if small leakage paths appear. But it has been found that the irregular shape of the jacket boot, particularly as it is folded to pass under the clamp, is such that small leakage paths may occur and require the use of a sealing compound to block and seal the boot irregularities. Elevated temperatures or solvent action may also cause such compound to migrate, leaving small leakage paths into the sensor head and inner sensor chamber.
The subject invention represents a means for achieving reliable sealing of the top sensor head, and in a manner that is versatile and allows accommodation of the wide range of conditions under which resistance-tape sensors are used.
Accordingly, it is an object of this invention to develop positive compressive forces for engaging and holding tightly together the components that comprise the top end termination.
Another object is to provide means for making the compressive forces adjustable and for allowing increase in compressive force to meet application conditions such as wide swings in operating temperature.
A further object is to provide a sensor top head termination that mechanically clamps the lower sensor elements so as to reliably support their weight and to resist pull down forces which may be imparted by turbulence or by solids present in slurries or liquids being gauged.
Another object is to provide a sensor top head which clamps and seals adequately against the breather tube and the electric leadwires emerging from the top end of the sensor.
Another object is to provide a sensor top head of modular design so that materials of construction can be changed or varied within the head to accommodate the gauged materials and operating conditions, and also to allow reduction in the component cost of the head structure.
Another object is to provide a head which allows the attachment of a lifting handle at the top of the sensor and a close-fitting channel to engage and protect the lower sensor along its length.
A further object is to provide a top head structure of such envelope dimensions that it can be used within, and retrofitted into, existing sensor mounting arrangements.
Another object of this invention is to provide a sensor head which can be conveniently disassembled and remade if a flaw should be discovered during the manufacturing process.
A further object is to provide a top head structure which can be later disassembled after a period of use, certain components cleaned, replaced or upgraded, and the sensor reassembled and returned to service.
Another object is to provide a sensor top head which may be composed of metallic or polymeric materials, or of coated elements or composite materials, so as to allow optimization of performance at economical cost.